A typical example of such drones is the AR.Drone from Parrot SA, Paris, France, which is a quadricopter fitted with various sensors (three-axis accelerometers and gyros, altimeter, vertically-directed camera) and with an automatic system for stabilizing the drone in hovering flight, on the principles described in WO 2009/109711 (Parrot). The drone is also provided with a front camera picking up an image of the scene towards which the drone is heading.
The automatic stabilization system serves in particular to enable the drone to reach an equilibrium point automatically, and once the equilibrium point has been reached, to provide the corrections needed for maintaining the stationary point by trimming, i.e. by correcting small variations involving movements in translation due to external effects such as movements of the air and drift of the sensors.
The drone is piloted by a user using a remote-control device referred to herein as an “appliance”, which appliance is connected to the drone by a radio connection such as a WiFi (IEEE 802.11) or Bluetooth (registered trademarks) type wireless local area network.
In particularly advantageous manner, the drone may be piloted by means of a remote-control appliance that has a touch screen, wireless transceiver means, and two-axis inclination sensors constituting sensors for sensing the attitude of the housing of the appliance relative to a reference vertical direction associated with a terrestrial frame of reference. The screen of the appliance reproduces the image of the on-board front camera as transmitted over the wireless connection, together with various piloting and command symbols that are superposed on the image so as to enable the commands to be activated by the user causing a finger to make contact with the touch screen.
The user has two different possible modes of piloting, either of which modes can be activated at will.
The first piloting mode, referred to below as “autopilot mode” is the default piloting mode, and it implements the independent and automatic stabilization system of the drone. The movement of the drone is defined as being a movement from one stable state to another stable state, with the corresponding change of the stationary point being obtained by independent commands (the simplest autopilot mode consisting in merely servo-controlling the drone at a stationary point).
The second piloting mode, referred to below as the “reactive mode”, is a mode in which the drone is piloted more directly by the user, by means of a combination:                firstly of signals issued by the inclination detector of the appliance; for example, in order to make the drone advance, the user tilts the appliance about the corresponding pitching axis, and in order to move the drone to the right or the left the user tilts the same appliance relative to the roll axis; and        secondly commands that are available on the touch screen, in particular an “up/down” command (corresponding to a throttle command) and a “turn left/right” command (causing the drone to pivot about a yaw axis).        
The changeover from autopilot mode to reactive mode is obtained by pressing a finger on a specific symbol displayed on the touch screen. Pressing the finger on this symbol causes the reactive mode to be activated immediately, and the reactive mode remains activated so long as finger contact is maintained at that location. Piloting is then obtained by tilting the appliance and/or manipulating “up/down” and turn “left/right” symbols that are displayed on the touch screen.
Autopilot mode (e.g. implemented in the manner described in above-mentioned WO 2009/109711) is activated as soon as the user removes the finger from the corresponding symbol.
That method of proceeding allows piloting to be entirely effective and intuitive; nevertheless, it presents several drawbacks.
A first drawback lies in the fact that in order to deactivate autopilot mode and switch to reactive mode piloting, the user must look at the screen of the appliance in order to put a finger accurately on the location where the symbol corresponding to the command that is to be activated is located (activate/deactivate autopilot mode; up/down; turn left/right about a yaw axis).
The user generally perceives this obligation as being a constraint, since the user must stop looking at the drone, precisely at the moment when taking over control in order to pilot the drone in reactive mode, which is awkward since the drone is piloted at sight and it is not good for the user to glance away from it. It is much easier to control the movements of the drone while looking at it, rather than looking at the video returned by the on-board camera, which does not show certain obstacles that lie outside the field of view of the camera (ceiling in an enclosed space, etc.).
In any event, although reactive mode includes commands that are intuitive (piloting by tilting the appliance about two axes), it nevertheless requires continued observation of the screen for the up/down and yaw axis turn commands (piloting by pressing on symbols displayed on the touch screen).
A second drawback stems from the fact that to perform the intuitive piloting commands by tilting the appliance one way or the other about two axes, it is necessary to hold the appliance level. Proceeding in that way is awkward, particularly outdoors, since a reflection of the sun can prevent the screen of the appliance being seen properly; to avoid such reflection, the user tilts the appliance, but then it is no longer possible to change over to reactive mode since the appliance is no longer horizontal.
A third drawback stems from the fact that reactive mode piloting requires simultaneous use of two fingers, using a finger of the left hand to activate right/left rotation commands and a finger of the right hand to activate up/down commands (or vice versa). If the user seeks to actuate some other function, e.g. an order to shoot (virtually) by pressing on a “fire” symbol displayed on the touch screen, then it is necessary to let go one of the commands in order to use the corresponding finger to press on that symbol.
A fourth drawback stems from the fact that five touch zones are defined solely for piloting, and the simultaneous use of two fingers often masks messages or information that might be essential for the user.
A fifth drawback stems from the fact that left-handed and right-handed users are treated differently since the commands are not symmetrical.
A sixth drawback stems from the fact that the drone is often an element in a video game in which the player pilots the drone and must simultaneously perform game actions, e.g. shooting at virtual or real targets. The user must then activate the shoot command very quickly, and almost instinctively, even though the user's attention is absorbed by piloting, the user's hands are busy, and the user is looking at the drone.
Other command or piloting interfaces are described for example in EP 1 621 238 A1 (emulating a joystick type pointer device at the point of contact of a stylus or finger on a touch screen) or in US 2007/077541 A1 (piloting by acting on a stylus or a finger on a steering wheel displayed on a touch screen). However neither of those proposals is capable of mitigating the above-mentioned drawbacks.